Process for determining the conditions needed in controllably obtaining sulfate pulp having a predetermined kappa number

ABSTRACT

A process is provided for determining the conditions needed in controllably obtaining a predetermined degree of delignification and therefore a predetermined Kappa number in the manufacture of sulfate pulp. A sample is taken of the alkaline pulping liquor at an early stage in the pulping of wood of the type to be pulped, the sample is titrated with an acid to the end point at which the conductivity of the sample has decreased to a relatively constant value, the NaOH (g/l) concentration is determined, and from this NaOH concentration at the desired Kappa value in the finished sulfate pulp the &#39;&#39;&#39;&#39;H&#39;&#39;&#39;&#39; factor is determined, which prescribed the pulping time and temperature relationship needed to obtain pulp of this Kappa value. Then the pulping time and cooking temperature are controlled during the pulping in accordance with such &#39;&#39;&#39;&#39;M&#39;&#39;&#39;&#39; factor.

United States Patent 11 1 Noreus 51 May 27, 1975 [75] Inventor: Sture Erik Olof Noreus, Husum,

Sweden [73] Assignee: Mo Och Domsjo Aktiebolag,

Ornskoldsvik, Sweden 221 Filed: on. 2, 1973 [21] Appl. No.: 402,697

Related US. Application Data [63] Continuation-impart of Ser. No. 142226, May 1l 1971, abandoned.

[30] Foreign Application Priority Data May 15, 1970 Sweden 6795/70 [56] References Cited UNITED STATES PATENTS 8/1960 Brown et al .1 324/65 R 1/1971 Rivers 162/49 OTHER PUBLICATIONS Vroom, The H Factor: A Means of Expressing Cook- Conductivity ing Times and Temperatures as a Single Variable," Pulp & Paper Mac. of Can. Vol 58, No. 3 (1957) p. 228-231.

Rahkonen, V., Simulator Controlled Batch Cooking of Sulphate Pulp," Proc. Xlllth EUCEPA Conference, Torremolinos (1970) Primary Examiner-56 Leon Bashore Assistant ExaminerAlfred DAndrea, Jr.

[57] ABSTRACT A process is provided for determining the conditions needed in controllably obtaining a predetermined degree of delignification and therefore a predetermined Kappa number in the manufacture of sulfate pulp A sample is taken of the alkaline pulping liquor at an early stage in the pulping of wood of the type to be pulped, the sample is titrated with an acid to the end point at which the conductivity of the sample has decreased to a relatively constant value, the NaOH (g/l) concentration is determined, and from this NaOl-l concentration at the desired Kappa value in the finished sulfate pulp the H factor is determined, which prescribed the pulping time and temperature relationship needed to obtain pulp of this Kappa value. Then the pulping time and cooking temperature are controlled during the pulping in accordance with such M factor.

23 Claims, 10 Drawing Figures ml Acid Added leg. sulfuric acid) FATENYES EAT 27 ms SHEET B IO |2 l4 l6 I8 20 22 mlH SO (2.5 N)

FIG. /0

l4 l6 I8 20 IO l2 mLH SO (2.5 N)

F/G. IE

PATENTED HAY 2 7 I975 NaOH SHEET 3 .886034 FIG. /-I

2 E g 1 U l 5 o ml Acid Added (e. g. sulfuric acid) FIG. I 2 A /A LEGEND A DIFFERENTIAL CONDUCTIVITY CONDUCTOMETRIC TITRATION AA 0 A L?! l l l I i l l l l i I4 l6 I8 20 22 24 2s NGOH Concentration Measured by Usual Laboratory Method Q/L FATEHTEU MY 2? ms "H" factor Q I l q I J QNQOH/L (A) F/GZ DPO-No "H"- factor FIG. 3

PROCESS FOR DETERMINING THE CONDITIONS NEEDED IN CONTROLLABLY OBTAINING SULFATE PULP HAVING A PREDETERMINED KAPPA NUMBER This application is a continuation-in-part of Ser. No. 142,226, filed May ll, l97l, and now abandoned.

The objective of a pulping process is to free the cellulose fibers from the wood, with the least amount of damage to the cellulose content thereof. For this purpose, in the sulfate pulping process the wood is treated at a high temperature with an alkaline cooking liquor consisting substantially of sodium hydroxide and sodium sulfide in order to extract the lignin from the wood, and thereby free the cellulose fibers. This process is called delignification. Delignification also makes it possible to obtain a strong cellulose pulp having a high degree of brightness and durability, after the cellulose pulp has been treated with bleaching chemicals.

The degree of delignification that is required depends upon the use to which the sulfate pulp is to be put, although the cost of the raw materials also plays an important part. Thus, in the manufacture of fully bleached sulfate pulp, the desired degree of dilignification is determined in part by the cost of the wood and the bleaching chemicals, and in part by the technical properties of the pulp for the end use required, such as m the manufacture of paper. Generally, the greater the degree of delignification obtained in the digester, the less bleaching chemicals are needed to obtain a certain degree of brightness in the pulp. The resulting reduction in chemicals consumption, however, is counteracted by an impaired pulp yield, and a reduction in the strength of the pulp. On the other hand, if the degree of delignification is less, the result is an increase in the cost of bleaching chemicals, and at the same time an Increase in the volume of chemical waste, which is undesirable from the standpoint of care and protection of the environment.

By taking into account the desired quality of the pulp, the actual cost of raw materials, and the care and protection of the environment, it is possible to determine the degree of delignification required to obtain the maximum economic advantage in pulping effienemy. The degree of delignification is determined by the quality of the wood, the amount of chemicals charged to the system per ton of dry wood, the woodlo-liquid ratio, and the intensity of the pulping condit ons, i.e., the digestion or cooking time, and the digestion or cooking temperature.

By the quality of the wood is meant primarily how much alkali is required to delignify the pulp to a specific Kappa number under standardized conditions. The requisite amount of alkali is dependent on the condition of the wood, such as the proportion of knots, and bark content, as well as on the chemical composition of the wood. It is, however, difficult to determine prior pulping how much alkali is required for pulping any particular batch of wood, and consequently it is difficult to determine in advance how much alkali is needed to obtain pulp of a desired Kappa number.

The amount of chemicals charged to the system per ton of dry wood is primarily determined by three variables, namely, the amount of dry wood charged to the system, the amount of white liquor charged to the system, and the composition and strength of the white liquor. Of these, the quality of white liquor charged to e system and the strength and composition of the liquor can be determined with a fair degree of accuracy, but the quantity of dry wood in the digester is much more difficult to determine accurately. There are several reasons for this, the most important being the diff"- culty in estimating the moisture content of the chips, and the variations in volumetric weight of the wood, i.e., its density, and the degree of packing of the chips in the digester. Consequently, it is practically impossible to establish with any degree of accuracy how much dry wood is in the digester for any given pulping batch, which means that rough estimates have to suffice. The moisture content of the wood chips and the quantity of dry wood in the digester also serve as large sources of error in determining the wood-to-liquid ratio.

The intensity of the pulping process greatly influences the delignification. It is therefore important to maintain good control over both temperature and time during the pulping. Thus, batch pulping processes have normally been carried out in accordance with a fixed time and temperature schedule, the degree of delignification obtained with pulp in a pilot run using a specific pulping process being used as a measure to determine the requisite amount of alkali for a number of succeeding batches, to obtain a desired degree of delignification. This procedure, however, results in pulp of nonuniform quality, because the wood is nonuniform from batch to batch.

Efforts have been made to improve the uniformity in the degree of delignification by maintaining a close control over the variables which affect the delignification. However, it has been found extremely difficult to measure the moisture content of the chips accurately, and there is no useful method for measuring the quality of the wood starting material. Indirect methods such as determining the lignin content of the pulping liquor or its pH have not been successful, owing to the inaccurate gauges which are available.

U.S. Pat. No. 3,553,075 to Rivers, dated Jan. 5, 1971, proposed to control the hydroxide concentration of the pulping liquor by determining the differential conductivity of the liquor at spaced stages of the pulping process, and adjusting the hydroxide concentration from a previously prepared curve of conductivity against hydroxide ion concentration. The objective is to monitor alkali concentration during the pulping. The conductivity is measured before and after neutralization, and kept at a desired level throughout the pulping by adding white or black liquor, or adjusting pulping temperature and/or time. However, this method by determining differential conductivity does not give any better result than a direct conductivity measurement on the varying mill pulping liquor and it is not possible to obtain a uniform pulping capable of producing pulp of a pre-selected Kappa number by this method.

In accordance with the invention, a process is provided for determining with considerable accuracy the conditions required for any desired degree of delignification, and thus make it possible to reproducibly prepare sulfate pulps of uniform quality. In the process of the invention, the wood is pulped to a desired Kappa value under pulping conditions established on he asi of H factor determined from a graph of H factor against Kappa value over a range of alkalinities (in terms of g/ NaOl-l) corresponding to the alkalinities required for the pulping of the type of wood selected. A family of such curves, one for each of a series of alkalinities within such range, serves as the reference graph. The

alkalinity of one or more samples taken at an early stage from a pulping liquor used to pulp the same type of wood is determined by titration with an acid, to an end point determined as the limiting relatively constant value of the conductivity of the sample that is reached as conductivity decreases during the acid titration. Thus, the alkalinity of the sample obtained by this measurement establishes the curve of the reference graph applicable to this sample of wood, and from this curve that is thus selected, the l-I" factor applicable to obtain a sulfate cellulose pulp having a predetermined Kappa value is read off. The H" factor in turn establishes pulping time and/or pulping temperature for the selected degree of delignification.

In the process of the invention, to obtain a sample of alkaline liquor for the determination, the sulfate pulping is begun in the conventional manner, by charging and thoroughly mixing wood chips and alkaline pulping liquor in the digester. A sulfate pulping liquor, as is well known, is an aqueous solution of alkali, usually NaOH, and Na s. The pulping is then begun, and allowed to continue for an initial pulping period during which at least 20 percent of the alkali added initially up to about 85 percent of the alkali added initially, preferably from 40 percent to 75 percent, has been consumed. after which a sample of the pulping liquor is taken, and titrated with an acid to the end point determined as the limiting conductivity of the sample. Thus, either a grad ually or rapidly increasing temperature during the initial pulping can be used as desired but approximately the same rate of increase would be used afterwards as before. The determination is usually valid only for ini tial heating rates and temperatures approximating those used in obtaining the sample.

In preparing the pulping sample for titration, the rate of temperature increase during the initial pulping states can be within the range from about O.lC/minute to about 25C/minute, preferably from about 0.5 to about lOC/minute.

In the titration of strong alkali with strong acid, following the progress of the titration by following the conductivity of the acid base mixture, it is found that the conductivity decreases as the amount of free alkali decreases, until the neutral point (pl-I7) is reached. at which point the conductivity of the mixture is at a minimum. With addition of more acid although the pH continues to decrease, as the mixture becomes more acid, the conductivity again increases. The shape of curve that is obtained is shown in FIG. 10 at page 465 of the text Principles of Physical Chemistry, by Maron and Prutton.

The usual course of a conductometric titration does not involve titration and measuring conductivity to the end point. Because the curve follows a straight-line course down to the end point on each side, several measurements can be made on each side of the end point sufficient to determine the slope of the curve, and the curves extrapolated to their point of intersection, which is the end point.

This is not the course that the conductivity follows when an alkaline sulfate pulping liquor is titrated by acid. The conductivity does not reach a minimum and then begin to rise again at once. Quite unexpectedly, the conductivity decreases, reaches a plateau where it remains constant for some time, and then begins to rise again. A typical curve is shown in FIG. l-l. In FIG. ll, the solid line shows the actual curve followed by conductivity during the titration, according to the acid added. The dashed line shows the extrapolation of the curves to the theoretical end point, which in fact is entirely theoretical, since it is never reached.

Thus, in accordance with the invention the curve obtained when graphing conductivity as a function in the quantity of acid charged to the sample is asymptotic, and very characteristic. At first, as acid is added, conductivity decreases rapidly, but after a certain quantity of acid has been reached (which quantity is dependent on the alkalinity of the samples), the change in conductivity with addition of more acid becomes very small, or disappears altogether. The point at which the change in conductivity diminishes or disappears constitutes the end point, and this point is marked by a change in slope of the curve. This point is referred to as the limiting value in the conductivity, and is the point where conductivity becomes relatively constant.

Determination of alkali concentration in alkaline pulping liquor reproducibly and with a degree of accuracy is very difficult. It is essential in the determination of the invention to ascertain the alkali consumed in the pulping, to the stage at which the sample is taken, but it is not possible to do this directly, using a conductivity meter. A conductivity meter does not give an accurate reading as an absolute value because many ions con tribute to conductivity that do not aid or even affect the pulping. For this reason, it has been quite surprising and unexpected that this can be determined by acid titration, to the end point at which conductivity reaches an approximate constant level, and that such alkali concentration makes it possible to determine the H factor required for obtaining sulfate pulp of any desired Kappa value, reproducibly and accurately. Consequently, the acid titration technique and the selected end point are quite important features of this invention.

It is unique to find an alkaline liquor which when titrated with acid decreases in conductivity to a constant value. Prior to this discovery, this characteristic appears not to have been known to be true of alkaline sulfate pulping liquor. The procedure for the conductometric titration of pulping liquor that is given in TAPPI calls for the usual determination of points on either side of the end point, and extrapolation of the curves to their point of intersection. If this be done, of course the plateau is missed altogether, and if perchance the points determined happen to be on the plateau, the end point determined by extrapolation is of course erroneous. This may explain why conductometric titrations of alkaline sulfate pulping liquor have given nonreproducible results.

The pH at which the conductivity of alkaline sulfate pulping liquor reaches a plateau at which it has a constant value is still on the alkaline side, and not as low as 7. It can in fact be considerably higher than 7, on the alkaline side ranging from about 9 to about 13. Nonetheless. although this point is not the neutralization point, it is still a satisfactory end point (which is also surprising). since if one arbitrarily selects the alkali concentration indicated by the amount of acid required to reduce the conductivity to a constant value, one finds that this concentration can be utilized as a sure guide to the selection of appropriate pulping conditions for the remainder of the pulping. Moreover, this concentration can be determined with an accuracy of: 0.4 gram per liter. measured as NaOH, so that the method is clearly exact even though it does not determine total OH ion concentration, but only the OH ion concentration at the plateau level, which is less. Use of the point at which the plateau is reached, and not the extrapolated end point, has the further advantage that at no time during the titration is the pH ever on the acid side. One can simply stop the titration while the pH is still alkaline. This means that one can avoid the difficulty of determining points on the acid side, since the sulphate pulping liquor at an acid pH liberates H 8, and lignin precipitates, creating problems and inaccuracies. On the alkaline side, none of these reactions occur.

The accuracy of this method is in contrast to the accuracy of a method for determining total OH concentration based on differential conductivity, such as is disclosed in Rivers US. Pat. No. 3,553,075. The problem is that differential conductivity, being an absolute conductivity value determination, reflects the presence of any ion that conducts a current. Since there are other ions present in alkaline pulping liquor in addition to OH, it is not possible to accurately determine total OH concentration in this way. In fact, a series of comparative analyses using the titration technique of this invention and the differential conductivity method of Rivers has shown that the differential conductivity method is accurate only within the range of i 4.0 grams per liter, measured as NaOH.

The degree of variation using these analytical techniques was demonstrated by a comparative series of experiments conducting analyses of 88 samples of black liquor, having a concentration of NaOH within the range from 12 to 26 grams per liter. The results for analyses selected at random from the test series are shown in FIGS. l-2. The triangles represent values determined by the Rivers differential conductivity technique. The solid circles represent values obtained by the conductometric titration technique of this invention. It is apparent that the conductometric titration technique is far more accurate, in the determination of alkali.

It may be noted that the principle used by Rivers is entirely different from the principle used in an acid titration. Acid titration gives a direct determination of OH, because OH are converted to HOH by the addition of hydrogen ion Hi In this technique, the conduc' tivity of the solution is used as an indication of the amount of acid added, and of OH taken up in the form of water. On the other hand, the Rivers technique uses the difference between the conductivity of the alkaline liquor before the addition of the acid, and after neutralization with the acid. It thus gives an absolute value based on conductivity, and this value of course reflects not only OH ions but also any other ions present which also conduct a current. Thus, the Rivers method and the method of this invention are quite different in principle, and they are also different in the accuracy of the alkali determination that is obtained thereby.

FIG. 1-1 represents the curve followed by conductivity during the titration of alkaline sulfate pulping liquor by acid.

FIGS. l-2 compares the conductrimetric titration technique of this invention with the differential conductivity technique of Rivers, US. Pat. No. 3,553,075.

FIGS. 1A to IE represent typical acid titration curves obtained in accordance with the process of the invention, in which conductivity is graphed against the amount of acid added. The point at which conductivity reaches or approaches a limiting value is taken as the end point, and is marked P in the Figures. There is one figure for each working Example 1A to IE, and the data in the Figures are taken from the working Examples 1A to IE, as will be seen from the disclosure thereof, infra.

FIG. 2 represents a graph of H" factor against percent alkali as NaOH.

FIG. 3 represents a reference graph, showing a family of curves of H" factor against Kappa number for a series of alkali concentrations ranging from l2.5 g/l. to 40 g/l. NaOH.

FIG. 4 is a schematic view of apparatus useful in carrying out the process of the invention.

The device or meter by which the conductivity is determined is not critical. Any convenient conductivitymeasuring device or meter can be used. One useful conductivity meter is provided with a reference electrode, for example the Kemotron four-electrode type, which registers electrical conductivity at different acid charges.

The acid employed in the titration is an organic or inorganic acid, preferably an inorganic acid, and preferably an acid which is nonoxidizing under the titration conditions. The acid is used in dilute aqueous solution. The normality of the solution is not critical, and can be within the range from about 0.1 to about 6N. Preferred acids are sulfuric acid and hydrochloric acid. Sulfuric acid has the advantage of a high sulfur content, which corresponds to the pulping liquor. Other inorganic acids such as orthophosphoric acid, hydrobromic acid, hydroiodic acid, metaphosphoric acid and pyrophosphoric acid also can be used, as well as organic acids such as acetic, formic, trichloroacetic, and propionic acids. Strong oxidizing acids such as persulfuric acid and nitric acid may be used under some conditions, but usually should be avoided.

The amount of acid added during the titration to the end point corresponds to the amount of alkali present, and the latter can therefore be determined by calculation from the amount of acid. The alkali content is calculated as NaOH in g/l.

The alkali concentration makes it possible to select the correct curve to determine H factor for a given (desired) Kappa value on the reference graph. The reference graph is composed of a family of curves, one for each alkali concentration (NaOH in g/l) at which a pulping can be carried out over the entire range of useful alkali concentrations. One reference graph is set up for each type of wood to be digested, for instance, spruce, fir, pine, birch, eucalyptus, beech, oak, maple, aspen, cedar, hemlock, cherry, chestnut, locust, elm, and the curves are based on the Kappa values obtained for pulps processed at given H" factors in the digester to be used. Thus, each plant would establish its own reference graph empirically, based on actual pulping experience for the type of wood to be pulped.

After the correct curve for the determined alkali concentration has been ascertained, the H" factor for the Kappa value of pulp desired can be read off, and from the H" factor the pulping temperature and pulping time can be ascertained.

The I-I" factor corresponds to a unit of pulping, and represents the number of hours of pulping at C. At a higher temperature, more units of pulping can be completed within a given time, and at a lower temperature, less. Thus, H factor is a measure of how much pulping is needed at 100C, or at temperatures above and below 100C.

In fact, any pulping temperature can be used in the process of the invention, within the range from about 1 10 to about 180C, and the pulping times also can be widely varied, from about 1 minute to about 10 hours, preferably from about 160 to about 180C. for from about minutes to about 3 hours. The H factor determines how long the pulping must be at a selected temperature, and vice versa, for a given Kappa value, at the alkali concentration determined in the titration.

1'1" factor is described by Vroom, Pulp and Paper Magazine of Canada 1957, pages 228 to 231.

The first step in the development of the H" factor by Vroom was the establishment of relative reaction rate values corresponding to a range of temperature levels. Vroom quite arbitrarily chose the reaction rate at 100C. as unity, and rates at all other temperatures were related to this standard. The Arrhenius equation was used in the form where k reaction rate,

T temperature in degrees absolute, and

B and A are constants.

The value for A was based on the work by Larocque and Maass, Canadian Journal of Research, B19z1-16 (1941). Then, at the arbitrarily chosen rate of unity at 100C, the equation becomes and the relative rate at any other temperature is given y Tables of these reaction rate values can then be prepared for any desired temperature range:

TABLE 1 RELATIVE RATE VALUES FOR THE H" FACTOR IN SULFATE PULPING TABLE l- Continued RELATIVE RATE VALUES FOR THE H" FACTOR 1N SULFATE PULPING Relative Temper Relative Temper- Relative Temper- Rate ature C Rate ature C Rate ature "C 5 l 14 96 144 1279 174 5 115 105 145 1387 175 6 116 114 146 1503 176 7 117 126 147 1629 177 7 118 138 148 1766 178 8 119 150 149 1914 I79 9 120 165 150 2042 180 10 121 182 151 2213 181 11 122 197 152 2398 182 12 123 217 153 2600 183 14 124 236 154 2818 I84 15 125 260 155 3054 I85 17 126 281 156 3258 186 18 12' 305 157 3531 187 20 128 336 158 3827 188 22 129 364 159 4082 189 Employing these relative rate values, a curve of rate against time in hours can be plotted for any cooking cycle, and the area under such a curve is designated as the H factor.

The H" factor represents the number of units of digestion per hour at 100C. The total number of digestion units needed, the H factor value from the reference graph curve, can be obtained using the above table as a multiple of the lower number of units per hour at lower temperatures, or as a fraction of the higher number of units per hour at higher temperatures.

As a simplified example, let it be assumed that the H factor indicated by the reference graph curve is 401. Then, the desired Kappa value will be obtained after the equivalent of a 1 hour pulping at 160C, or a 2 hour pulping at 152C, or a 3 hour pulping at 147C; or a 12 hour pulping at 168C. This is an oversimplification because as a practical matter, however, the pulping is not carried out solely at the temperature of the Table, but over a gradual heating to the pulping temperature, and the H" factor represents the units of digestion over the entire pulping cycle. Thus, the comelali e Temper- Relative Temper- Relative Temperom Heated, and in fact the Rate ature c Rate a Rate am": 0 putatlon is slightly more c p h H" factor for any pulping cycle represents t e area I 100 25 150 under a relative reaction rate versus time curve. Thus, l 182 ii :3? {2; the H" factor determines the shape of any of an 1nfil 103 34 133 51 1 163 nite number of curves that can be used for a given pulp- 18g 37 134 563 12 4 mg 1 135 610 H 2 106 45 136 661 166 As a further example, let it be assumed that the H 2 107 49 137 716 167 factor is 1594. To obtain such an H" factor value, one 2 108 54 138 777 168 k h th risin tem- 3 109 139 855 169 can use a pulplng cycle of 1 ours in e g g 110 66 140 927 170 perature stage from C to 170C, and 1% hours at 111 73 141 1005 171 o shown b t e 4 112 79 142 1039 172 170 C m the final pulping stage. This 15 y 4 I13 7 143 1180 173 55 following computation:

TABLE 11 Relative I Time 1 Time from Temp. rate of Average nterva N u start (hours) "C reaction rate X (hours) H factor 0 0.25 X A l 9 X if. 2 15 0,75 4 X V. m

1.00 66 [63 X V z 4' 60 1.25 2 594 X V M9 TABLE I] Continued Relative Time Time from Temp. rate of Average Interval start (hours) "C reaction rate X (hours) "H" factor 927 X 1 V2 l39l Total i594 'Calculatcd to the nearest whole number.

In the above calculation, in the rising temperature stage of the cycle, the relative rate values have been averaged over A hour periods. While of course this is an approximation, it is satisfactory for most purposes. More accurate approximations can be obtained by taking smaller time intervals, or other methods such as Simpsons rule or the trapezoidal rule may be employed.

Thus, any conditions of pulping temperature and time which give the H" factor that has been determined can be used.

In a continuous or in a batch process, it is possible to adapt the sampling operation to the particular digester being used, for example, using continuous sulfate digesters of the type having two pulping zones, with the pulping sequence regulated in said zones. Even in the case of batch pulpings, more than one sample can be taken, if suitable, in order to have a greater control over the pulping sequence. However, it has been found that the pulping sequence can be established fully from the results provided by a single sample measured in accordance with the invention.

In a conventional pulp digestion, the same temperature and time schedule is used for all pulpings, from day to day, for each batch, in a batch operation, or continuously, in the continuous operation. In the process of the invention, in contrast, the pulping conditions for each batch, in a batch operation, or continuously, in a continuous operation, are varied according to the H" factor determined for the particular lot of work being pulped, as shown by the sample.

Such variation can be effected in pulping temperature or in pulping time, or both, and this type of variation is the usual one, but it is also possible to adjust the alkali concentration by adding either water, black liquor or alkali to move to a different alkali concentration curve in the reference graph, and so obtain a more favorable or more convenient H" factor. It may be desirable, but it is not essential, to take another sample, if more alkali is added, since the presence of a higher alkali concentration may effect the wood in a different way. If the additional alkali is added at a later stage of the pulping, however, the effect is minimal, and another sample is unnecessary.

The apparatus shown in FIG. 4 is designed for a continuous pulping process. The chips are fed through a preheater 2 where they are heated by steam and hot gases led by lines 25, 22 from a digester 1, and gas evaporator 20, and are then passed continuously via the high-pressure feeder l and the line 3 into the digester l by means of pulping liquor circulating in the line 11. The excess of chips and pulping liquor, if any, is recirculated by line 12. The digester is a long reactor through which the chips and liquor progress at a steady rate. The temperature is adjusted at the steam-heated heaters 4 of the fluid taken from the pulping zones 5 and circulated via lines 6, 7, 8, 9 to achieve the desired rate of temperature increase and pulping temperature.

Liquor sample lines goes from line 8, 9 to an analyzer 13 for alkali which analyzer meters the conductivity of the liquor. The meter signals a computer (not shown) 5 which is programmed to adjust the temperature at the steam-heated heaters 4, and in this way control the pulping by prescribed variations in pulping temperature.

Provision also is made to adjust alkali by adding 0 white and black liquor via lines l4, 15 if this appears desirable.

The pulp is removed at the bottom of the digester and is fed to the blow tank via valve 16 and line 17. Spent black liquor is led to the recovery plant via lines l8, l9 and the evaporators 20, 21, while the hot gases from the evaporators are led by lines 22, 23, 24 to the condensers and preheater.

The pulp has a substantially constant Kappa number,

30 due to the control of the pulping conditions in accordance with the invention.

The following Examples in the opinion of the inventors represent preferred embodiments of the invention.

5 EXAMPLE 1 In this Example, an alkali determination is made at an initial pulping stage to ascertain what l-l" factor to apply for pulp of a given Kappa number. After the al- 0 kali determination has been made, one proceeds using the known H factor correlation with Kappa number to determine the pulping conditions needed for pulp of a given Kappa number. Since this correlation is dependent on alkali concentrations the known family of 5 curves is prepared for the pulping equipment, showing the variations between H factor and Kappa number at different alkali concentration, From these relation ships, it is possible to make a single curve showing the H" factor at different alkali concentrations needed in 50 order to reach a certain Kappa number.

Alkali charge active alkali (as NaOH) Sulfidity Wood-to-liquid ratio kg/l Rate of temperature increase in the initial stage from to l'lOC O.5C/min.

65 When the digester had been heated to a temperature of approximately 140C, a ml sample of the pulping liquor was taken, and titrated with 2.5 N aqueous sulfuric acid, while measuring the conductivity using a conductivity meter. Acid addition was continued until conductivity reached a constant value. The conductivities were graphed, and the limiting value was shown as the change in stage as the end point was reached at P, as shown in FIGS. 1A to IE7 The alkalinity (as NaOH, g/l) determined in the live pulping liquors was as follows:

Batch No. A B C D E NaOH g/l 21.0 19.7 17.5 16.6 15.0

This batch digester had previously been used to establish the reference graphs shown in FIGS. 2 and 3 by carrying out a series of digestions, using pine wood chips, under the same pulping conditions, i.e., a pulping liquor to wood ratio of 3 5, a rate of temperature increase from 80C to the pulping temperature of 0.5C/min., and a final pulping temperature of 170C, and the Kappa numbers of the pulps obtained noted. Kappa number was graphed against H" factor, for each alkali concentration from 12.5 g/l. to 49 g/l. NaOH, and the reference graph appears as FIG. 3. Alkali concentration was also graphed against H" factor, and this graph appears as FIG. 2.

Pulping conditions for each of batches A to E are readily determined to produce pulp of a desired Kappa number by selecting the curve in FIG. 3 corresponding to the alkali concentration of the batch, selecting the Kappa number, and then reading the H factor off the curve.

The relationship of the curves of FIG. 3 can be expressed not only graphically as shown in the curves but also mathematically by the empirical formula:

K A K, 6 Kappa number N X "#3 10 where k 0.1806 A is the alkali concentration k, 80.7 as NaOH in g/l. k 0.9388 H is the "H" factor.

The values of k k and k depend on the rate of increase in temperature before the sample is taken, and the pulping temperature and time at which the sample is taken, and have to be determined empirically for each profile of pulping conditions. However, the formula is applicable for all pulping conditions that can be used.

The H' factors to obtain a pulp having a Kappa number of 33 at this alkalinity were determined from the selected curves of FIG. 3 for each of batches A to E (a mathematical computation using the above empirical formula gives the same result, since the formula is derived from the curves):

Batch No. A B C D E H factor 1740 1870 2160 2300 2650 Batch No. A B C D E Time at 170C in minutes 90 98 1 17 126 172 The indicated pulping conditions were then applied. The cook was heated to the maximum temperature of 170C, and this temperature was maintained for the five separate batches for 90, 98, 117, 126 and 172 minutes, respectively, as projected. The pulping process was then interrupted. and the pulps obtained were screened and washed, after which their Kappa numbers were determined. The following results were obtained:

Batch No. A B C D E Kappa number 32.1 33.3 32 32.8 33.5

It is apparent that in each case the Kappa number ob tained very closely approximated the projected Kappa number of 33.

For the purpose of comparison, a second series of five pulpings were made, using the same type of chips, and the same pulping conditions as those used in the tests described, with the exception that the pulping was held at the maximum temperature for minutes in each case, as would normally have been done, in the absence of the determination in accordance with the invention. Upon completion of the pulping process, the resulting pulps were screened and washed, after which their Kappa numbers were estimated. The following results were obtained:

Batch No. F G H l I Kappa number 26.1 28.5 32.0 34.5 39.2

It is evident from these tests that the pulps had very different Kappa numbers, showing that the standardized conditions were not capable of giving a uniform result. On the other hand, the process of the invention made it possible to overcome such differences, and produce in each case a uniform quality of pulp having a predetermined degree of deligniflcation.

EXAMPLE 2 Pine chips from the same batch and pulping liquor were charged to the same batch digester as that used in Example 1, and pulped under the following conditions:

Alkali charge 24% alkali (as NaOH) Sulfidity 30% Wood-to-Iiquid ratio 1:3.5 kg/l Rate of temperature increase,

80 to 170C 0.5C/min.

Maximum temperature 170C After the temperature of the cook had reached C, a sample of the pulping liquor was taken, and the alkali content of the liquor was determined by the conductometric titration to be 29.5 g. of NaOl-I per liter. At a Kappa number of 33, the 30 g/l NaOH curve in FIG. 3 gives an H" factor of 1200, and the rate Table shows this to correspond to a cooking time of 61 minutes at C. Upon completion of the pulping under the conditions prescribed, the pulp was washed and screened. The Kappa number of the pulp obtained was 31.5, very closely approximating the projected value.

For comparison purposes, pine chips taken from the same batch of chips were pulped for 2 hours at a maximum temperature of 170C. The conditions in other respects were the same as those described above. The Kappa number of the pulp obtained was l9.9, which is far from the desired value.

This shows that the method of the invention gives control over the variations in alkali charge caused among other things by nonuniformity in the quantity of wood and alkali. When pulping according to a standard schedule, a Kappa number of 19.9 was obtained, which means that the strength of the pulp was particularly impaired, while at the same time the pulp yield decreased.

EXAMPLE 3 Example 2 was repeated, using pine chips taken from the same batch of chips as that used in Example 2. When the temperature of the pulping had reached 140C, a sample of the pulping liquor was taken, and the H factor required to obtain a pulp having a Kappa number of 33 was determined to be I300, using the formula above. This l-l" factor corresponds to a pulping time of I20 minutes, at a maximum temperature of 164C. The process was then carried out as described, and when the temperature of the pulping had reached 164C, the temperature was maintained for 120 minutes, after which the pulping was interrupted. The pulp obtained by this pulping had a Kappa number of 32, which was satisfactorily close to the projected value of 33.

Having regard to the foregoing disclosure, the following is claimed as the inventive and patentable embodiments thereof:

1. A process for determining the conditions needed in controllably obtaining a predetermined degree of delignification and therefore a predetermined Kappa number in the manufacture of sulfate pulp from wood, and then pulping the wood under the conditions thereby determined, using an alkaline pulping liquor comprising an alkali metal hydroxide and an alkali metal sulfide, which comprises taking a sample of alkaline pulping liquor at an early stage in the pulping of the wood to be pulped at which from at least 20 percent up to about 85 percent of the alkali added initially has been consumed, titrating the sample with an acid to the end point at which the conductivity of the sample has decreased to a relatively constant value, determining from the amount of acid added during the titration the alkali concentration at this end point, and from this alkali concentration determining the H" factor at the desired Kappa value in the finished sulfate pulp, and from the H" factor determining the pulping time and temperature relationship needed to obtain pulp of this Kappa value, and then pulping the wood at the determined time and temperatures to obtain pulp of this Kappa value.

2. A process according to claim 1, in which the sample is prepared by mixing wood chips and alkaline sulfate pulping liquor.

3. A process according to claim 1, in which the pulping is carried out in the early stage at a rising temperature within the range from about O.l to about 25C per minute.

4. A process according to claim 1, in which the acid employed in the titration is a strong inorganic acid which is nonoxidizing under the titration conditions.

5. A process according to claim 1, in which the acid is used in dilute aqueous solution whose normality is within the range from about 0.1 to about 6 N.

6. A process according to claim 1, in which the alkaline pulping liquor is brought to a temperature within the range from about 1 10C to about 180C, and then the sample is taken.

7. A process according to claim 1, in which the titration is effected with a strong inorganic acid.

8. A process according to claim 7, in which the acid is selected from the group consisting of sulfuric acid and hydrochloric acid.

9. A process according to claim 1, in which the pulping is carried out batchwise.

10. A process according to claim 1, in which the pulping is carried out continuously.

11. A process according to claim 1, in which the alkali metal hydroxide is sodium hydroxide.

12. A process for determining the alkali concentration of an alkaline pulping liquor comprising an alkali metal hydroxide and an alkali metal sulfide, which comprises taking a sample of the alkaline pulping liquor at a stage at which from at least 20% up to about of the alkali added initially has been consumed, titrating the sample with an acid to the end point at which the conductivity of the sample has decreased to a relatively constant value, and determining the alkali concentration at the end point from the amount of titrating acid added to the end point.

13. A process according to claim 12, in which the alkali metal hydroxide is sodium hydroxide, and the alkali metal sulfide is sodium sulfide.

14. A process for preparing sulfate pulps of relatively uniform quality having a desired Kappa number which comprises pulping wood using an alkaline pulping liquor comprising alkali metal hydroxide and alkali metal sulfide at a pulping temperature and for a pulping time established by taking a sample of alkaline pulping liquor at an early stage in the pulping of the wood to be pulped at which from at least 20 percent up to about 85 percent of the alkali added initially has been consumed, titrating the sample with an acid to the end point at which the conductivity of the sample has decreased to a relatively constant value, determining from the amount of acid added during the titration the alkali concentration at this end point, and from this alkali concentration determining the H factor at the desired Kappa value in the finished sulfate pulp, and from the *H" factor determining the pulping time and temperature relationship needed to obtain pulp of this Kappa value.

15. A process according to claim 14, in which the alkali concentration of one or more samples taken at an early pulping stage from the alkaline pulping liquor is determined by titration with an acid to an end point corresponding to the limiting relatively constant value of the conductivity of the sample that is reached as conductivity decreases during the acid titration.

16. A process according to claim 14, in which to obtain a sample of alkaline liquor for the H factor determination, the sulfate pulping is begun by charging and mixing wood chips and alkaline pulping liquor in a digester, the pulping is then begun, using an increasing temperature, and allowed to continue for an initial pulping period during which from at least 20 percent up to about 85 percent of the alkali added initially has been consumed, after which a sample of the pulping liquor is taken, and titrated with an acid to the end point, and then the pulping is carried out at approximately the same rate of temperature increase during the initial stages until the final pulping temperature is reached.

17. A process according to claim 16, in which the rate of temperature increase during the initial pulping stages is within the range from about 0. lClminute to about 25C/minute.

18. A process according to claim 17 in which the rate of temperature increase during the initial pulping stages is within the range from about 05 to about lOC/minute.

19. Apparatus for the continuous preparation of sulfate pulps of relatively uniform quality having a desired Kappa number, comprising, in positive flow connection and in combination, a digester arranged for the through passage of particulate wood while converting the wood in transit to pulp by pulping with alkaline sulfate pulping liquor, means in fluid flow communication with the digester for introducing wood and alkaline pulping liquor at one end of the digester, means in fluid flow communication with the digester for withdrawing wood pulp at another end of the digester, means for heating the digester, means in fluid flow communication with the digester for withdrawing a sample of pulping liquor at a position along the digester corresponding to an early stage of pulping at which from at least 20 percent 16 up to about percent of the alkali added initially has been consumed, means in fluid flow communication with the digester for measuring the conductivity of the sample, means in fluid flow communication with the digester for titrating the sample with acid to an end point at which conductivity has been decreased to a relatively constant value, and means connected with the digester for controlling the temperature of the digester and the rate of the transit of the digesting wood therethrough according to the alkali concentration of the sample, and an H" factor at that alkali concentration.

20. Apparatus according to claim 19, including a preheater for heating the particulate wood prior to passage through the digester.

21. Apparatus according to claim 19, including a gas evaporator for concentrating spent alkaline liquor, and means for recycling hot gases from the evaporator to a preheater for the particulate wood.

22. Apparatus according to claim 19, in which the means for determining conductivity is a conductivity meter.

23. Apparatus according to claim 19, including means for adding alkali to the liquor in the digester at a position along the digester corresponding to a later stage of the digestion.

l i i '0' =0 

1. A PROCESS FOR DETERMINING THE CONDITIONS NEEDED IN CONTROLLABLY OBTAINING A PREDETERMINED DEGREE OF DELIGNIFICATION AND THEREFORE A PREDETERMINED KAPPA NUMBER IN THE MANUFACTURE OF SULFATE PULP FROM WOOD, AND THEN PULPING THE WOOD UNDER THE CONDITIONS THEREBY DETERMINED, USING AN ALKALINE PULPING LIQUOR COMPRISING AN ALKALI METAL HYDROXIDE AND AN ALKALI METAL SULFIDE, WHICH COMPRISES TAKING A SAMPLE OF ALKALINE PULPING LIQUOR AT AN EARLY STAGE IN THE PULPING OF THE WOOD TO BE PULPED AT WHICH FROM AT LEAST 20 PERCENT UP TO ABOUT 85 PERCENT OF THE ALKALI ADDED INITIALLY HAS BEEN CONSUMED, TITRATING THE SAMPLE WITH AN ACID TO THE END POINT AT WHICH THE CONDUCTIVITY OF THE SAMPLE HAS DECREASED TO A RELATIVELY CONSTANT VALUE, DETERMINING FROM THE AMOUNT OF ACID ADDED DURING THE TITRATION THE ALKALI CONCENTRATION AT THE END POINT, AND FROM THIS ALKALI CONCENTRATION DETERMINING THE "H" FACTOR AT THE DESIRED KAPPA VALUE IN THE FINISHED SULFATE PULP, AND FROM THE "H" FACTOR DETERMINING THE PULPING TIME AND TEMPERATURE RELATIONSHIP NEEDED TO OBTAIN PULP OF THIS KAPPA VALUE, AND THEN PULPING THE WOOD AT THE DETERMINED TIME AND TEMPERATURES TO OBTAIN PULP OF THIS KAPPA VALUE.
 2. A process according to claim 1, in which the sample is prepared by mixing wood chips and alkaline sulfate puLping liquor.
 3. A process according to claim 1, in which the pulping is carried out in the early stage at a rising temperature within the range from about 0.1* to about 25*C per minute.
 4. A process according to claim 1, in which the acid employed in the titration is a strong inorganic acid which is nonoxidizing under the titration conditions.
 5. A process according to claim 1, in which the acid is used in dilute aqueous solution whose normality is within the range from about 0.1 to about 6 N.
 6. A process according to claim 1, in which the alkaline pulping liquor is brought to a temperature within the range from about 110*C to about 180*C, and then the sample is taken.
 7. A process according to claim 1, in which the titration is effected with a strong inorganic acid.
 8. A process according to claim 7, in which the acid is selected from the group consisting of sulfuric acid and hydrochloric acid.
 9. A process according to claim 1, in which the pulping is carried out batchwise.
 10. A process according to claim 1, in which the pulping is carried out continuously.
 11. A process according to claim 1, in which the alkali metal hydroxide is sodium hydroxide.
 12. A process for determining the alkali concentration of an alkaline pulping liquor comprising an alkali metal hydroxide and an alkali metal sulfide, which comprises taking a sample of the alkaline pulping liquor at a stage at which from at least 20% up to about 85% of the alkali added initially has been consumed, titrating the sample with an acid to the end point at which the conductivity of the sample has decreased to a relatively constant value, and determining the alkali concentration at the end point from the amount of titrating acid added to the end point.
 13. A process according to claim 12, in which the alkali metal hydroxide is sodium hydroxide, and the alkali metal sulfide is sodium sulfide.
 14. A process for preparing sulfate pulps of relatively uniform quality having a desired Kappa number which comprises pulping wood using an alkaline pulping liquor comprising alkali metal hydroxide and alkali metal sulfide at a pulping temperature and for a pulping time established by taking a sample of alkaline pulping liquor at an early stage in the pulping of the wood to be pulped at which from at least 20 percent up to about 85 percent of the alkali added initially has been consumed, titrating the sample with an acid to the end point at which the conductivity of the sample has decreased to a relatively constant value, determining from the amount of acid added during the titration the alkali concentration at this end point, and from this alkali concentration determining the ''''H'''' factor at the desired Kappa value in the finished sulfate pulp, and from the ''''H'''' factor determining the pulping time and temperature relationship needed to obtain pulp of this Kappa value.
 15. A process according to claim 14, in which the alkali concentration of one or more samples taken at an early pulping stage from the alkaline pulping liquor is determined by titration with an acid to an end point corresponding to the limiting relatively constant value of the conductivity of the sample that is reached as conductivity decreases during the acid titration.
 16. A process according to claim 14, in which to obtain a sample of alkaline liquor for the ''''H'''' factor determination, the sulfate pulping is begun by charging and mixing wood chips and alkaline pulping liquor in a digester, the pulping is then begun, using an increasing temperature, and allowed to continue for an initial pulping period during which from at least 20 percent up to about 85 percent of the alkali added initially has been consumed, after which a sample of the pulping liquor is taken, and titrated with an acid to the end point, and then the pulping is carried out at approximately the same rate of temperature increase duriNg the initial stages until the final pulping temperature is reached.
 17. A process according to claim 16, in which the rate of temperature increase during the initial pulping stages is within the range from about 0.1*C/minute to about 25*C/minute.
 18. A process according to claim 17 in which the rate of temperature increase during the initial pulping stages is within the range from about 0.5* to about 10*C/minute.
 19. Apparatus for the continuous preparation of sulfate pulps of relatively uniform quality having a desired Kappa number, comprising, in positive flow connection and in combination, a digester arranged for the through passage of particulate wood while converting the wood in transit to pulp by pulping with alkaline sulfate pulping liquor, means in fluid flow communication with the digester for introducing wood and alkaline pulping liquor at one end of the digester, means in fluid flow communication with the digester for withdrawing wood pulp at another end of the digester, means for heating the digester, means in fluid flow communication with the digester for withdrawing a sample of pulping liquor at a position along the digester corresponding to an early stage of pulping at which from at least 20 percent up to about 85 percent of the alkali added initially has been consumed, means in fluid flow communication with the digester for measuring the conductivity of the sample, means in fluid flow communication with the digester for titrating the sample with acid to an end point at which conductivity has been decreased to a relatively constant value, and means connected with the digester for controlling the temperature of the digester and the rate of the transit of the digesting wood therethrough according to the alkali concentration of the sample, and an ''''H'''' factor at that alkali concentration.
 20. Apparatus according to claim 19, including a preheater for heating the particulate wood prior to passage through the digester.
 21. Apparatus according to claim 19, including a gas evaporator for concentrating spent alkaline liquor, and means for recycling hot gases from the evaporator to a preheater for the particulate wood.
 22. Apparatus according to claim 19, in which the means for determining conductivity is a conductivity meter.
 23. Apparatus according to claim 19, including means for adding alkali to the liquor in the digester at a position along the digester corresponding to a later stage of the digestion. 